The zeta potential data confirmed the adsorption of the large compact Keggin Si(W12O40)](4-) and heteropolytungstate (LST) anions on both alpha-Al2O3. The adsorbed [Si(W12O40)](4-) ions should form a thick steric barrier, similar to 1 nm, but this effect was not reflected in the yield stress results. A relatively strong additional attractive force explained the increasing maximum yield stress. A similar result was observed with LST. For the spherical alpha-Al2O3 the largest maximum yield stress (at zero charge) occurred at 0.6 dwb% additive. This attractive force was attributed to patch charge attraction between the negative additive patch and the positive surface site. For the low surface area platelet alpha-Al2O3 suspensions, complete surface coverage occurred at a low additive concentration as reflected by an almost identical all negative zeta potential-pH behaviour at all additive concentrations at low pH. The slightly larger yield stress at pH 2.5 could be due to this patch charge bridging attraction. (C) 2019 Elsevier B.V. All rights reserved.